In circuits driving inductive loads it is often a concern to control the inductive flyback when a load is turned off. FIG. 1 is a prior art circuit 10 that illustrates a MOS transistor 12 driving an inductive load 14 in a high side driver configuration. A diode 16 is connected in parallel with load 14 as a "free-wheeling diode" to recirculate the current I from load 14 and limit the node voltage at output node OUT to a diode voltage drop (typically between 0.5-1.5 V depending upon the type of diode utilized) below circuit ground when MOS transistor 12 is turned off. Circuit 10 suffers from two problems: first, as load 14 recirculates its current I (because current cannot instantaneously change within an inductive load) the power dissipation is dictated by V.sub.D16 *I where V.sub.D16 is the forward biased voltage of diode 16 and I is the recirculation current. V.sub.D16 is a fairly large voltage (approximately 0.5-1.5 V) causing an undesirably high power dissipation. This creates excessive heat and in some cases may limit the type of packages with which circuit 10 may be manufactured and may also limit the operating temperature range of circuit 10. A second problem is substrate current injection and can be seen in FIG. 2. FIG. 2 is a circuit cross-section of prior art circuit 10. When transistor 12 turns off, inductive load 14 (not shown) experiences inductive flyback which drives output node OUT negative. When output node OUT reaches a diode voltage drop below circuit ground (which is the potential of the P- substrate) the p-n junction formed at the P epi layer and the N- tank forward biases and current is pulled from the substrate. Since the substrate is resistive a voltage gradient develops along the substrate. This voltage gradient may be of large enough value such that N type tanks within the P type substrate "de-bias", thus creating potential "latch-up" conditions. Furthermore, de-biasing may cause loss of data in CMOS logic circuits. In addition to the de-biasing problem, substrate current injection can lead to latch-up and increased power dissipation.
FIG. 3 is another prior an circuit 20 that utilizes a "free-wheeling diode" 22. MOS transistor 24 is connected to diode 22 and inductive load 14 which are connected together in parallel. Circuit 20 is connected in a low side driver configuration. When transistor 24 turns off output node OUT tries to increase to a large positive voltage due to the inductive flyback of load 14. Diode 22 clamps OUT to only a forward diode voltage drop above Vdd. Diode 22 then recirculates the current I until load 14 dissipates all of its stored energy. Circuit 20 suffers from the same problems and limitations as discussed for circuit 10 of FIG. 1. During recirculation, the power dissipation is high (V.sub.D22 *I) and circuit 20 experiences substrate current injection, however, circuit 20 experiences substrate current injection in a different manner. FIG .4 explains this clearly. If the drain (tied to OUT) of transistor 24 increases to a diode drop above Vdd, a parasitic PNP transistor 26 is turned on and begins to conduct. Parasitic PNP transistor 26 is formed from P epi, the N- tank (the cathode of diode 22) and the P- tank (the anode of diode 22). This parasitic PNP action may cause significant power dissipation due to large V.sub.CE and substrate de-biasing with its potential problems as discussed in circuit 10 of FIG. 1.
It is an object of this invention to provide a circuit solution that recirculates the current of an inductive load during load turn-off such that power dissipation is reduced and the substrate current injection is eliminated. Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings herein.